This invention relates to the field of fluid flow velocity measurement.
Various types of flowmeters have been utilized in the past for measuring the time of flight of an acoustic signal between a pair of spaced transducers. Typical examples of such systems are U.S. Pat. Nos. 4,452,090; 3,901,0778; 3,738,169; and 4,611,496. In general, acoustic or ultrasonic pulses are transmitted either alternately or simultaneously in the downstream and the upstream directions between two transducers. The travel time in the downstream direction and in the upstream direction is determined and the two resulting values are combined to either determine the flow velocity value or the speed of sound in the fluid medium. This information can also be used to determine the composition of a binary gas mixture assumed to be motionless. Both analog and digital systems have been designed for this purpose, with varying degrees of success.
Known devices of the above type suffer from several disadvantages. Firstly, many of the analog devices are highly susceptible to signal noise and drift, particularly in those applications in which the environment is quite noisy and subject to changing ambient conditions. Although digital devices tend to be less susceptible to noise, lack of stability is still a problem with such devices. In addition, most known devices suffer from a limited dynamic range which is not well suited to a wide variety of applications or to particular applications in which the flow velocities are subject to wide variation (e.g., in a spirometry application). Further, known devices suffer from a limited measurement accuracy due to a variety of factors, including phase changes or frequency changes in the acoustic signal during transmission and reception, between sample changes of a magnitude greater than the ability of the measurement apparatus to unambiguously detect, and relatively low resolution capability.